I learned about longitudinal motion, reading a Charles de Kunffy book, and I’ve been teaching it ever since. The horse’s top line gets longer and shorter at the walk and at the canter. So as dressage riders since we ride on contact, we have to follow with our elastic elbows…. The struggle is we are so busy trying to get the horse on the bit that we forget to also follow. One of the nuances of dressage how to ask and allow at the same time.:-)